The invention relates to wireless communications systems. In particular, the invention relates to acquisition of a pilot signal in a wireless communication system.
A wireless communication system may comprise multiple remote units and multiple base stations. FIG. 1 exemplifies an embodiment of a terrestrial wireless communication system with three remote units 10A, 10B and 10C and two base stations 12. In FIG. 1, the three remote units are shown as a mobile telephone unit installed in a car 10A, a portable computer remote 10B, and a fixed location unit 10C such as might be found in a wireless local loop or meter reading system. Remote units may be any type of communication unit such as, for example, hand-held personal communication system units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment. FIG. 1 shows a forward link 14 from the base station 12 to the remote units 10 and a reverse link 16 from the remote units 10 to the base stations 12.
Communication between remote units and base stations, over the wireless channel, can be accomplished using one of a variety of multiple access techniques which facilitate a large number of users in a limited frequency spectrum. These multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). An industry standard for CDMA is set forth in the TIA/EIA Interim Standard entitled xe2x80x9cMobile Stationxe2x80x94Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, TIA/EIA/IS-95, and its progeny (collectively referred to here as IS-95), the contents of which are incorporated by reference herein in their entirety. Additional information concerning a CDMA communication system is disclosed in U.S. Pat. No. 4,901,307, entitled SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR
TERRESTRIAL REPEATERS, (the ""307 patent) assigned to the assignee of the present invention and incorporated in its entirety herein by reference.
In the ""307 patent, a multiple access technique is disclosed where a large number of mobile telephone system users, each having a transceiver, communicate through base stations using CDMA spread spectrum communication signals. The CDMA modulation techniques disclosed in the ""307 patent offer many advantages over other modulation techniques used in wireless communication systems such as TDMA and FDMA. For example, CDMA permits the frequency spectrum to be reused multiple times, thereby permitting an increase in system user capacity. Additionally, use of CDMA techniques permits the special problems of the terrestrial channel to be overcome by mitigation of the adverse effects of multipath, e.g. fading, while also exploiting the advantages thereof.
In a wireless communication system, a signal may travel several distinct propagation paths as it propagates between base stations and remote units. The multipath signal generated by the characteristics of the wireless channel presents a challenge to the communication system. One characteristic of a multipath channel is the time spread introduced in a signal that is transmitted through the channel. For example, if an ideal impulse is transmitted over a multipath channel, the received signal appears as a stream of pulses. Another characteristic of the multipath channel is that each path through the channel may cause a different attenuation factor. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different signal strength than other received pulses. Yet another characteristic of the multipath channel is that each path through the channel may cause a different phase on the signal. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different phase than other received pulses.
In the wireless channel, the multipath is created by reflection of the signal from obstacles in the environment such as, for example, buildings, trees, cars, and people. Accordingly, the wireless channel is generally a time varying multipath channel due to the relative motion of the structures that create the multipath. For example, if an ideal impulse is transmitted over the time varying multipath channel, the received stream of pulses changes in time delay, attenuation, and phase as a function of the time that the ideal impulse is transmitted.
The multipath characteristics of a channel can affect the signal received by the remote unit and result in, among other things, fading of the signal. Fading is the result of the phasing characteristics of the multipath channel. A fade occurs when multipath vectors add destructively, yielding a received signal that is smaller in amplitude than either individual vector. For example if a sine wave is transmitted through a multipath channel having two paths where the first path has an attenuation factor of X dB, a time delay of xcex4 with a phase shift of "THgr" radians, and the second path has an attenuation factor of X dB, a time delay of xcex4 with a phase shift of "THgr"+xcfx80 radians, no signal is received at the output of the channel because the two signals, being equal amplitude and opposite phase, cancel each other. Thus, fading may have a severe negative effect on the performance of a wireless communication system.
A CDMA communication system is optimized for operation in a multipath environment. For example, the forward link and reverse link signals are modulated with a high frequency pseudonoise (PN) sequence. The PN modulation allows the many different multipath instances of the same signal to be separately received through the use of a xe2x80x9crakexe2x80x9d receiver design. In a rake receiver, each element within a set of demodulation elements can be assigned to an individual multipath instance of a signal. The demodulated outputs of the demodulation elements are then combined to generate a combined signal. Thus, all of the multipath signal instances must fade together before the combined signal experiences a deep fade.
In a communication system based on the industry standard for CDMA, IS-95, each of the multiple base stations transmits a pilot signal having a common PN sequence. Each base station transmits the pilot signal offset in time from neighboring base stations so that the signals can be distinguished from one another at the remote unit. At any given time, the remote unit may receive a variety of pilot signals from multiple base stations. Using a copy of the PN sequence produced by a local PN generator, the entire PN space can be searched by the remote unit. Using the search results, the controller distinguishes pilot signals from multiple base stations based on the time offset.
In the remote unit, a controller is used to assign demodulation elements to the available multipath signal instances. A search engine is used to provide data to the controller concerning the multipath components of the received signal. The search engine measures the arrival time and amplitude of the multipath components of a pilot signal transmitted by the base stations. The effect of the multipath environment on the pilot signal and the data signal transmitted by a common base station is very similar because the signals travel through the same channel at the same time. Therefore, determining the multipath environment""s effect on the pilot signal allows the controller to assign demodulation elements to the data channel multipath signal instances.
The search engine determines the multipath components of the pilot signals of base stations in the proximity of the remote unit by searching through a sequence of potential PN offsets and measuring the energy of the pilot signal received at each of the potential PN offsets. The controller evaluates the energy associated with a potential offset, and, if it exceeds a threshold, assigns a signal demodulation element to that offset. A method and apparatus of demodulation element assignment based on searcher energy levels is disclosed in U.S. Pat. No. 5,490,165 entitled DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS, (the ""165 patent) assigned to the assignee of the present invention.
FIG. 2 shows an exemplifying set of multipath signal instances of a single pilot signal from a base station arriving at a remote unit. The vertical axis represents the power received in decibels (dB). The horizontal axis represents the delay in the arrival time of a signal instance due to multipath delays. The axis (not shown) going into the page represents a segment of time. Each signal spike in the common plane of the page has arrived at the remote unit at a common time but has been transmitted by the base station at a different time. Each signal spike 22-27 has traveled a different path and therefore exhibits a different time delay, a different amplitude, and a different phase response. The six different signal spikes represented by spikes 22-27 are representative of a severe multipath environment. A typical urban environment produces fewer usable paths. The noise floor of the system is represented by the peaks and dips having lower energy levels. The task of the search engine is to identify the delay, as measured by the horizontal axis, and amplitude, as measured by the vertical axis, of signal spikes 22-27 for potential demodulation element assignment.
Note, as shown in FIG. 2, each of the multipath peaks varies in amplitude as a function of time as shown by the uneven ridge of each multipath peak. In the limited time shown, there are no major changes in the multipath peaks. Over a more extended time range, multipath peaks disappear and new paths are created as time progresses. Multipath peaks are likely to merge together or blur into a wide peak over time.
Typically, the operation of the search engine is overseen by a controller. The controller commands the search engine to step through a set of offsets, called a search window, that is likely to contain one or more multipath signal peaks suitable for assignment to a demodulation element. For each offset, the search engine reports the energy it found offset back to the controller. Demodulation elements may then be assigned by the controller to the paths identified by the search engine (i.e. the timing reference of their PN generators is aligned with the timing of the identified path). Once a demodulation element has locked onto the signal, it then tracks that path on its own without controller supervision, until the path fades away or until the demodulation element is assigned to another path by the controller.
As noted above, each base station in a given geographical area is assigned a sequence offset of a common PN pilot sequence. For example, according to IS-95, a PN sequence having 215 chips and repeating every 26.66 milliseconds (ms) is transmitted by each base station in the system at one of 512 PN sequence offsets as a pilot signal. According to IS-95 operation, the base stations continually transmit the pilot signal which can be used by the remote unit to identify the base station as well as other functions, such as for example, determining the multipath environment the remote unit is operating in and synchronization of remote unit timing to the base station timing.
During initial power on, or any other situation when the remote unit has lost a pilot signal such as when performing a hard hand-off to a different operating frequency, the remote unit evaluates all possible PN offsets of the pilot PN sequence. Typically, a search engine measures the pilot signal strength at all possible PN offsets, proceeding at a measurement rate that produces an accurate measure of the pilot signal present at the corresponding offset. Proceeding in this manner, the search engine determines the PN offset of base stations which are geographically near the remote unit. Searching each PN offset in this manner can take anywhere from hundreds of milliseconds to a few seconds depending on the channel conditions during acquisition. This amount of time for the remote unit to reacquire a pilot signal is detrimental to the remote unit operation, and may be annoying to the user of the remote unit.
FIG. 3 shows an extended portion of PN space on the horizontal axis. The groups of peaks 30, 32 and 34 represent transmissions from three different base stations. As shown, the signal from each base station signal experiences a different multipath environment. Also, each base station has a different PN offset from the PN reference 36. Thus, the controller may select a set of PN offsets corresponding to search windows for any of the identified base stations. This allows the remote unit to simultaneously demodulate signals from multiple base stations by assigning demodulation elements appropriately.
In a typical CDMA communication system, remote units sporadically establish bi-directional communications with a base station. For example, a cellular telephone remains idle for significant periods of time when no call is in process. However, to ensure that any message directed to a remote unit is received, the remote unit continuously monitors the communication channel, even while it is idle. For example, while idle, the remote unit monitors the forward link channel from the base station to detect incoming calls. During such idle periods, the cellular telephone continues to consume power to sustain the elements necessary to monitor for signals from the base stations. Many remote units are portable and are powered by an internal battery. For example, personal communication system (PCS) handsets are almost exclusively battery-powered. The consumption of battery resources by the remote unit in idle mode decreases the battery resources available to the remote unit when a call is placed or received. Therefore, it is desirable to minimize power consumption in a remote unit in the idle state and thereby increase battery life.
One means of reducing remote unit power consumption in a communication system is disclosed in U.S. Pat. No. 5,392,287, entitled APPARATUS AND METHOD FOR REDUCING POWER CONSUMPTION IN A MOBILE COMMUNICATION RECEIVER (the ""287 patent), assigned to the assignee of the present invention and hereby incorporated in its entirety herein by reference. In the ""287 patent, a technique for reducing power consumption in a remote unit operating in an idle mode (i.e. a remote unit which is not engaged in bidirectional communication with a base station) is disclosed. In idle, each remote unit periodically enters an xe2x80x9cactivexe2x80x9d state during which it prepares to and receives messages on a forward link communication channel. In the time period between successive active states, the remote unit enters an xe2x80x9cinactivexe2x80x9d state. During the remote unit""s inactive state, the base station does not send any messages to that remote unit, although it may send messages to other remote units in the system that are in the active state.
As disclosed in the ""287 patent, a base station broadcasts messages which are received by all remote units within the base station coverage area on a xe2x80x9cpaging channel.xe2x80x9d All idle remote units within the base station coverage area monitor the paging channel. The paging channel is divided in the time dimension into a continuous stream of xe2x80x9cslots.xe2x80x9d Each remote unit operating in slotted mode monitors only specific slots which have been assigned to it as assigned slots. The paging channel continually transmits messages in numbered slots, repeating the slot sequence, such as for example, every 640 slots. When a remote unit enters the coverage area of a base station, or if a remote unit is initially powered on, it communicates its presence to a preferred base station. Typically the preferred base station is the base station which has the strongest pilot signal as measured by the remote unit.
The preferred base station, along with a plurality of geographically near neighboring base stations, assign a slot, or a plurality of slots, within their respective paging channels, for the remote unit to monitor. The base station uses the slots in the paging channel to transmit control information to a remote unit, if necessary. The remote unit may also monitor a timing signal from the preferred base station allowing the remote unit to align, in the time dimension, to the base station slot timing. By aligning in the time dimension to the preferred base station slot timing, the remote unit can determine when a paging channel slot sequence begins. Thus, knowing when the paging channel slot sequence begins, which slots are assigned for it to monitor, the total number of slots in the repetitive paging channel sequence of slots, and the period of each slot, the remote unit is able to determine when its assigned slots occur.
Generally, the remote unit is in the inactive state while the base station is transmitting on the paging channel in slots which are not within the remote unit""s assigned set. While in the inactive state, the remote unit does not monitor timing signals transmitted by the base station, maintaining slot timing using an internal clock source. Additionally, while in the inactive state the remote unit may remove power from selected circuitry, such as, for example, circuits which monitor pilot signals transmitted by base stations to detect changes in the wireless channel including the search engine. Using its internal timing, the remote unit transits to its active state a short period of time before the next occurrence of an assigned slot.
When transiting to the active state, the remote unit applies power to circuitry that monitors the wireless channel, including the search engine. The search engine is used to reacquire the preferred base station""s pilot signal and to detect changes in the wireless channel which may have occurred due to the movement of the remote unit or to the movement of objects within the coverage area of the base station. In addition to reacquiring the pilot signal, the remote unit may perform any other actions or initializations in preparation of receiving a message at the beginning of its assigned slot.
When the remote unit enters the active state, it may receive messages in its assigned slots in the paging channel and respond to commands from the base station. For example, the remote unit may be commanded to activate a xe2x80x9ctrafficxe2x80x9d channel to establish a bidirectional communication link for conducting subsequent voice communication in response to an incoming call. If there is no message from the base station, or no command requesting the remote unit to remain active, at the end of the assigned slot the remote unit returns to the inactive state. In addition, the remote unit returns to the inactive state immediately if commanded to do so by the base station.
During its assigned slot, the remote unit""s search engine measures the pilot signal strength of the preferred base station as well as the pilot signal strengths of neighboring base stations. If the remote unit relocates from the coverage area of one base station to another neighboring base station""s coverage area, the remote unit needs to xe2x80x9chand-offxe2x80x9d to the neighboring base station. A hand-off occurs when the transmitted pilot signal strength of a neighbor base station becomes sufficiently stronger than the preferred base station. When this occurs, the neighboring base station is assigned as the preferred base station. Following a hand-off, in the next active state, the remote unit monitors the paging channel of the new preferred base station to receive messages and commands.
In addition to providing data for determining when a hand-off should occur, searches of the preferred base station""s pilot signal allow the remote unit to make adjustments to compensate for changes in the multipath environment.
For example, if one of the multipath signal instances weakens to the point that it is unusable, the remote unit may reassign demodulation elements accordingly.
Knowing the nominal PN offset of the preferred base station as well as a neighboring set of base stations, typically, the controller passes a set of search parameters to the search engine specifying PN offsets at which multipath signal instances of pilot signals are likely to be found. At the completion of the search, the search engine passes the search results to the controller. The controller analyzes the search results and selects a set of search parameters for the next search. Following selection of the new search parameters, the controller passes the parameters to the search engine and the search process is repeated. This process is repeated until the remote unit once again enters the inactive idle state.
Because searches occur only during the remote unit assigned slot, a limited time period is available for the searches to be performed. Increasing the number of searches performed by a remote unit would improve its ability to determine when a hand-off should occur, as well as providing more information about the multipath environment of the preferred base station, leading to a more robust communication system. However, remaining in the active state for a longer duration consumes more power and reduces the battery life of the remote unit.
Therefore, there is a need in the art for a method and apparatus to increase the efficiency and accuracy of the searching process of the remote unit without significantly increasing the power consumption.
The invention comprises a method and apparatus for searching in a remote unit, which provides for improved search efficiency in a slotted mode, wireless, communication system. In a slotted mode communication system, the remote unit may alternate between xe2x80x9cactivexe2x80x9d and xe2x80x9cinactivexe2x80x9d states to prolong battery life. In such a system, the remote unit enters the active state just prior to its assigned slot, returning to the inactive state following its assigned slot.
In one embodiment of the invention, the number of searches performed during an active state are increased by not evaluating the search results while the remote unit is in the active state. In this embodiment, a controller in the remote unit passes a selected set of search parameters to a search engine at the beginning of the assigned slot. The search engine performs a search, using the selected set of search parameters, and stores the search results in a data array. Immediately following completion of a search, the search engine interrupts the controller indicating the search is complete. Rather than first evaluating the results of the search to determine new search parameters, the controller immediately passes another set of search parameters to the search engine for the next search. Searching continues as long as the remote unit is in the active state. When the remote unit""s assigned slot has ended, or if commanded by the base station, the remote unit leaves the active state and enters the inactive state. During the inactive state the remote unit removes power from selected circuitry, thereby extending battery life. In addition, while in the inactive state the controller evaluates the results of the searches performed during the assigned slot of the previous active state. Evaluation of the search results provides a basis for the controller to generate new sets of search parameters for use by the search engine during a subsequent active state.
In another embodiment, the controller passes the search parameter sets to the search engine during a preparation period after the remote unit has reacquired the preferred base station. The preparation period occurs after the remote unit has entered its active state prior to its assigned slot. The search engine may then begin searching prior to occurrence of the remote unit""s assigned slot, thereby increasing the period available for searching, and increasing the number of searches that may be performed.
In yet another embodiment, a plurality of sets of search parameters are passed from the controller to the search engine during the preparation period or at the beginning of the assigned slot. In this embodiment, the search engine completes a search, stores the search results, and then proceeds to the next search without receiving additional commands from the controller. The search engine continues searching as long as the remote unit remains in the active state.
In one embodiment, after the remote unit enters the inactive state, the controller evaluates the stored search results. Following evaluation of the search results the remote unit remains in the inactive state unit the occurrence of its next active state. In another embodiment, after evaluation of the stored search results and the determination of a new set of search parameters, the controller enters a xe2x80x9cmore inactive state.xe2x80x9d In the xe2x80x9cmore inactive statexe2x80x9d additional circuitry in the remote unit has power removed, in addition to the circuitry which has had power removed in the inactive state. For example, after the controller has read the search results from memory, the memory may enter an inactive state in which it consumes less power. In addition, after the controller has evaluated the search results from the previous active state and selected new search parameters to be used during the subsequent active state, the controller may also become inactive until the next active state of the remote unit is entered. Removing power from additional circuitry further decreases power consumption in a remote unit and thereby increases battery life.